I was extremely moved by "A Canticle for Leibowitz" and recall one time driving north into New Hampshire on a solo camping expedition and hearing over the car radio the University of Southern New Hampshire broadcasting an episode of the radio version where the Abbot is getting Extreme Unction from an 'extra' head on the body of one of the "Pope's children".
The original work was complete in itself. I'm not aware of a follow-up volume. Was it a pre-quel? I'll look for it.

You might like the speculative fiction of R. A. Lafferty.

While I am not a Christian, A Canticle for Leibowitz is one of several works I feel are intrinsically Christian and moving and worthwhile: Among the others:

The Brothers Karamazov
Brideshead Revisited
The Master and Margarita